Maximizing the Output Power of Wind-Driven Grid-Connected ...

JKAU: Eng. Sci., Vol. 29 No. 1, pp: 39 – 49 (1439 A.H. / 2018 A.D.)

Doi: 10.4197/Eng. 29-2.3

39

Maximizing the Output Power of Wind-Driven Grid-Connected Cage Induction

Generator through Pole Control

M. A. Abdel-Halim and Abdulkarim Alghunaim

College of Engineering, Qassim University, Municipality of Qassim Region, Qassim, Saudi Arabia

[email protected]

Abstract. In this research, a cage induction generator has been linked to the grid and driven with a

wind-turbine to generate electrical power. The cage generator has been used in place of the costly

slip-ring generator. The performance characteristics of the cage induction generator have been

ameliorated through changing its number of poles to comply with the level of the wind speed to

maximize the mechanical power extracted from the wind. Pole changing has been achieved

employing pole-amplitude modulation technique resulting in three sets of pole numbers. The

results proved the feasibility and effectiveness of the suggested method, as the proposed technique

led to driving the generator, and consequently the wind-turbine at speeds close or equal to those

satisfying the optimum tip-speed ratio which corresponds to the point of maximum mechanical

power.

Keywords: Wind energy, Grid-connected Induction Generator, Pole-amplitude Modulation, Cage

Induction Generator.

List of Symbols:

Cp: Wind power coefficient

CQ: Wind torque coefficient

Ir: Rotor current

Is: Stator current

nm: Generator speed

ns: Synchronous speed

Vs: Stator voltage

Vw: Wind speed

PT: Turbine output power

Rr: Rotor resistance

Rs: Stator resistance

Rc: Core resistance

RT: Thevinin’s equivalence resistance

Rb: Radius of the turbine blade

Xr: Rotor leakage reactance

Xs: Stator leakage reactance

Xm: Magnetizing reactance

Zms :The equivalent series impedance

XT: Thevinin’s equivalence reactance

s: Per-unit slip

β: Blade pitch angle

λ : Tip-speed ratio

ωT: Angular speed of the wind turbine

τ: Generator induced torque

τT: Turbine output torque

1. Introduction

The utilization of wind energy is very

important and finds nowadays great interest.

This interest has become vital as the world is

going to face great shortage in the

conventional energy resources. Therefore,

generation of electrical energy from the

renewable wind energy has become an

essential goal of energy researches. The wind-

turbines as prime movers have variable and

unexpected velocities. If synchronous

generators are used, the frequency of the

40 M. A. Abdel-Halim and Abdulkarim Alghunaim

generated voltage will be variable as it is tied

to the prime-mover speed. Therefore, the need

to other types of generators or unconventional

solutions arises. Using of induction generators

solve the problem of these variable speed

turbines [1-3]

.

Most of the research works carried out in

this field aimed at maximizing the power

efficiency of the wind-driven induction-

generator by controlling the system such that

the turbine always extracts the possible

maximum wind-power at all wind speeds [4-10]

.

Shaltout, et al., [4]

proposed a simple

control strategy of a double fed induction

generator (DFIG) to track the maximum power

available at different wind speeds using a

rectifier-inverter set in the rotor. Abdel-halim

et al. [5]

proposed a technique for the rotor

injected voltage of a DFIG such that the wind

turbine tracks the maximum wind energy, and

at the same time the stator current is kept at

unity power factor. Boumassata et al.[6]

proposed a system constituting of a DFIG

using a controlled cycloconverter in the rotor

aiming at improving the maximum power

point tracking. Abdel-halim, et al. [7, 8]

suggested a method of controlling the

induction generator such that the driving wind-

turbine follows the point of maximum wind

power. This is employed through dual control

using a cycloconverter in the stator side and

three added resistances in the rotor side. Other

authors suggested controlling the IG from the

stator side using rectifier-inverter set [9]

with

the aim of tracing the maximum wind power

points, or using a cycloconverter [10]

to drive

the generator as near as possible to these

maximum points.

In general, control of the induction

generators is achieved via the use of solid-state

converters in the stator and/or the rotor sides.

When solid-state converters are used in the

rotor side, they will be of reduced rating

(about 25 % of the generator rating) [11]

. But in

this case, the induction generator should have

a slip ring rotor, which is expensive compared

to the cage one. To control the cheaper cage

induction generator utilizing rectifier-inverter

set, the converters should be used in the stator,

which is the only available side. In this case, if

a rectifier-inverter set is used, its rating should

be the same as that of the generator, which is

again costly. To avoid such costly converters,

another control techniques have to be used for

the cage induction generators such as changing

the number of poles.

Linni Jian, et al. [12]

proposed a novel

double-winding flux modulated permanent

magnet machine (FMPM) for stand-alone

wind power generation. Based on the flux-

modulating effect, a concentrated winding set

and a distributed winding set could be artfully

equipped on one stator component. The

authors showed that the proposed FMPM can

offer higher torque capability and stronger flux

adjustability than the existing single-winding

FMPMs generators.

It is noted that mostly the cage induction

generator has been used in constant speed

systems [3, 11]

. The present paper aims at

increasing the output electrical power of the

induction generators at the different wind

speeds through the control of the stator

number of poles. Pole control will be achieved

via changing the machine number of poles

using pole amplitude modulation technique for

the stator winding [13]

. The study will end with

suggesting control strategies to maximize the

output electrical power from the generator.

The research will be performed through

developing a circuit model for the controlled

grid-connected cage induction generator

considering pole-changing. Then, a

mathematical model will be developed for the

complete system. Based on this model, the

performance characteristics of the system will

Maximizing the Output Power of Wind-Driven Grid-Connected Cage Induction Generator through Pole Control 41

be computed, and a comprehensive analysis

for the performance characteristics of the

controlled generator will be carried out.

2. Studied System

The system under study comprises a

wind-driven grid-connected cage induction

generator controlled by pole amplitude

modulation technique. The generator is

directly linked to the grid (Fig. 1).

Fig. 1. The wind-driven grid-connected induction generator

system.

3. Control Method

To increase the output power of the

system, it is necessary to increase the extracted

power of the wind. This is achieved by driving

the generator at speed such that the turbine tip-

speed ratio is equal or very near to the

optimum value [14 & 15]

. Pole amplitude

modulation technique [13]

will be used to adjust

the generator speed at 3 levels. This technique

is done to extract the maximum wind power

through adjusting the turbine tip-speed ratio to

be close to its optimum value.

3.1 Pole-Amplitude Modulation (PAM)

In this technique the windings are

grouped in two sections, and the current

direction in one section of each phase is

reversed with respect to the other. This method

utilizes the norm of amplitude-modulation to

the space-distribution of the magnetic motive

force produced by the windings. The windings

design procedure is based on trigonometrical

equations [13]

.

In two-speed PAM windings the

connections of the three phase windings were

2-parallel-star/ series-delta [16,17]

. For three-

speed PAM windings, three speeds are made

possible by using 4-parallel-star/ 2-parallel-

star/ series delta switching [13]

, using the circuit

shown in Fig. 2. Table 1 explains how the

terminals are dealt with to get the 3 different

pole numbers that gives the 3 different speeds.

Fig. 2. Circuit diagram for 3 speed pole-amplitude

modulation.

Table 1. Different connections and supplying patterns for

the 3-speeds PAM.

Speed I (4-

Parallel-star)

Speed II (2-

Parellel-star)

Speed III

(Series-delta)

Join a₁ b₁ c₁ Join

a₃ b₃ c₃

Join a₂ a₄; b₂ b₄;

c₂ c₄

Supply a₂ a₄; b₂

b₄; c₂ c₄

Join a₁ b₁ c ₁

Isolate a₂ a₄ b₂ b₄

c₂ c₄

Supply a₃ b₃ c₃

Isolate a₂ a₃ a₄

b₂ b₃ b₄ c₂ c₃ c₄

Supply a₁ b₁ c₁

4. System Modelling

4.1 Wind Turbine Mathematical Model

Wind turbines are equipment specially

designed to convert amount of the energy of

the wind into useful mechanical energy.

Depending on the direction of the turbine axis,

wind turbines are classified into vertical-axis

and horizontal-axis ones [18]

. Nowadays,

almost all commercial wind turbines used in

conjunction with generators to deliver

electrical power to grids have horizontal-axis

two-bladed or three-bladed rotors.


The turbine output torque and power are

usually expressed in terms of non-dimensional

torque (CQ) and power (CP) coefficients as

follows [14, 15]

.

𝜏𝑇 =1

2𝜌𝜋𝑅𝑏

3𝐶𝑄(𝜆. 𝛽)𝑉𝑊2 (4.1)

𝑃𝑇 =1

2𝜌𝜋𝑅𝑏

2𝐶𝑃(𝜆. 𝛽)𝑉𝑊3 (4.2)

𝐶𝑄 = 𝐶𝑃 ∕ λ (4.3)

Note that the two coefficients are written

in terms of the pitch angle and the so-called

tip-speed-ratio; λ, defined as

λ = (ωT Rb) / Vw (4.4)

Where Rb is the radius of the turbine blade in

meter, ωT is the turbine angular velocity in

rad/s, and Vw is the wind speed in m/s. Fig. 3

shows typical variations of CQ and Cp for a

fixed-pitch wind turbine [15]

.

Fig. 3. Typical variations of CQ and CP for a fixed-pitch

wind turbine.

4.2 Induction Generator Circuit Model

Figure 4 shows a frequency-domain per-

phase circuit model of the 3-phase induction

generator [3]

. The equivalent circuit parameters

depend on the number of poles of the

generator [12]

.

The parameters of the equivalent circuit

depends on the number of stator poles which is

set through variation of the stator circuit

topology [13]

.

Fig. 4. Per-phase equivalent circuit of the induction

generator.

4.3 Induction Generator Mathematical Model

The induced torque of the induction

generator is related to applied voltage, slip and

the equivalent circuit parameters as follows [3]

:

𝜏 = 3 VT

2 Rr s⁄

ωs[(RT+Rr/s)2+ (XT+Xr)2] (4.5)

where VT, RT and XT are Thevenin's equivalent

circuit parameters of the stator. These are

given by

𝑉𝑇 = VsZms

√(Rs+Rcs)2+(Xs+Xms)2 (4.6)

𝑅𝑇 + j𝑋𝑇 = (Rs + jXs)// jXm // Rc (4.7)

where Rcs and Xms are the components of Zms

(The equivalent series impedance of the

parallel components; Rc and Xm).

5. Computation Algorithm

A computer program has been developed

to compute the performance of the induction

generator using pole-modulation-amplitude

when driven by wind turbine at the different

wind speeds. The computations have been

based on the mathematical models given in

Section 4.2. The algorithm followed during

computing the performance for the different

number of poles is as follows:

i- At a certain wind speed and number

of poles, the tip-speed ratio (λ) is initially

calculated assuming that the speed of

generator is equal to its synchronous speed

using Eqn. 4.4 Then, the torque coefficient

(CQ) is determined using the turbine


characteristic (Fig. 3), and the torque of the

wind turbine is calculated using Eqn. 4.1.

ii- The slip is calculated using Eqn. 4.5,

and a new speed of the generator is calculated.

Then, λ, CQ and the wind turbine torque are

recalculated. The iterative technique continues

until no variation in the slip occurs.

iii- At the calculated value of the slip,

the equivalent circuit (Fig. 4) is used to

calculate the generator currents, power factor,

output power, losses etc.

iv- The calculation is repeated for

different wind speeds over the wind speed

rang and for the three sets of poles.

6. Results and Discussion

The developed computer program has

been applied on a generator of the data given

in Appendix 1. The performance

characteristics at different number of poles,

namely; 4, 6 and 8 have been calculated. The

captured mechanical power from the wind is

affected by the number of poles (Fig. 5). As

the wind speed decreases, the number of poles

should be increased to extract more

mechanical power from the wind. For the

studied generator, for speeds higher than about

9.6 m/s 4-poles should be used, while when

the generator number of poles is changed to 6-

poles, more mechanical power is extracted

when the wind speed ranges between 9.6 m/s

and 6.8 m/s. To extract the highest mechanical

power at wind speeds lower than 6.8 m/s, the

number of poles should be changed to 8 poles.

The captured mechanical power from the

wind through pole control is very near to the

theoretical maximum extracted power from the

wind as depicted in Fig. 6.

The output electrical power from the

generator is more or less follow the same

manner as the input mechanical power (Fig.

7). The output electrical power is maximized

when the number of poles is set to 4-poles for

wind speeds higher than 9.2 m/s. Over the

wind speed range from 9.2 m/s and down to

about 7 m/s, the generator output electrical

power is the highest for 6-poles. At wind

speeds lower than 7 m/s, that of the 8-poles is

the highest.

The stator and rotor currents (Fig. 8& 9)

decrease generally as wind speed decreases.

An eye should be kept on these currents to

ensure that they are within the permissible

rated values. The investigation of these figures

show that if the number of poles is changed

corresponding to the specified wind speeds,

the stator currents will be kept within the safe

values (18.44 A for 4 poles, 14.98 A for 6

poles and 13.68 A for 8 poles).

The power factor curves (Fig. 10)

indicate that changing the number of poles as

the wind speed decreases below the middle

wind speed value (8 m/s) results in operation

at better power factors. At low wind speeds,

the power factor become negative as the

generated power is low to the extent that it

does not cover all the stator losses, and the

machine draws electrical power from the

supply to cover the rest of the losses.

7. Conclusions

Utilization of wind energy to generate

electrical power is very vital. Cage induction

generators- being cheap- can be used in small

wind energy conversion system to replace the

costly slip ring generators. To enhance the

performance of such induction generator, pole

amplitude modulation technique has been

applied to increase the captured wind

mechanical power, and hence the output

electrical power.

Pole amplitude modulation method with

three sets of poles has proved its effectiveness

in this regard. At high wind speeds, the

generator number of poles is set to the lowest

number, and at medium wind speeds the

number of poles is set to the medium value,


while at the low wind speed the highest

number of poles is used. Application of this

control technique ensures the operation of the

wind turbine around the maximum wind-

energy point at all wind speeds.

Fig. 5. Input mechanical power for the different number of poles versus wind speed.

Fig. 6. Input mechanical power using pole control compared with the theoretical extracted maximum power.

0

2000

4000

6000

8000

10000

12000

3 4 5 6 7 8 9 10 11 12

Inp

ut

Mec

han

ical

Po

we

r, W

Wind speed, m/s

4 poles6 poles8 polesTheoretical maximum power


Fig. 7. Output electrical power for the different number of poles versus wind speed.

Fig. 8. The rotor current versus the wind speed at the different number of poles.


Fig. 9. The stator current versus the wind speed at the different number of poles.

Fig. 10. Power factor versus the wind speed for different number of poles.


References

[1] Singh, Bhim (1995) “Induction Generators-A

prospective”, Electrical Machines and Power Systems,

23: 163-177.

[2] Subbiah, V. and Geetha, K. (1996) “Certain

Investigations on a Grid Connected Induction Generator

with Voltage Control”, Proc. of the IEEE International

Conference on Power Electronics, Drives and Energy

Systems, New Delhi, India, Jan. 1996, pp: 439-444.

[3] Akhmatov, V. (2005) "Induction Generators for Wind

Power", Multi-Science Publishing Co. Ltd., Essex,

United Kingdom.

[4] Shaltout, A.A. and El-Ramahi, A. F. (1995) “Maximum

power tracking for a wind driven induction generator

connected to a utility network”, Applied Energy, 52:

243-253.

[5] Abdel-halim, M. A., Mahfouz, A. A. and Almarshoud,

A. F. (2014) "Enhancing the Performance of Wind-

Energy-Driven Double-Fed Induction Generators",

Qassim University Scientific Journal- Engineering and

Computer Sciences, 7 (1: 23-41.

[6] Boumassata, A., Kerdoun, D., Cherfia, N. and

Bennecib, N. (2013) "Performance of Wind Energy

Conversion Systems Using a Cycloconverter to Control

A Doubly Fed Induction Generator", Energy Procedia,

Available online at www.sciencedirect.com, 42: 143–

152.

[7] Abdel-halim, M. A. (2015), “Dual Control of Induction

Generators for Tracing the Maximum Wind Energy”,

Journal of Engineering and Computer Sciences, Qassim

University, 8(1) (January): 21-44.

[8] Abdel-Halim, M. A., Mahfouz, A. A. and Almarshoud, A.

F. (2015) “Enhancing the Performance of a Stator and

Rotor Combined-Controlled Wind-Driven Induction

Generator”, JKAU: Eng. Sci., 26(1), DOI: 10.4197/Eng.

26-1.1, pp: 3- 23.

[9] Boldea, Ion (2016) “Variable Speed Generators”, 2nd

Edition, CRC Press, Taylor & Francis Group, Boca

Roton,.

[10] Ruben, P. (2001) “A Cage Induction Generator Using

Back to Back PWM Converters for Variable Speed Grid

Connected Wind Energy System”, Industrial Electronics

Conference (IECON), 2: 1376-1381.

[11] Abad, Gonzalo, Lo´pez, Jesu´s, Rodrı´guez, Miguel A.,

Marroyo, Luis and Iwanski, Grzegorz (2011) “Doubly

Fed Induction Machine- Modeling and Control for Wind

Energy Generation,” IEEE Press, Published by John

Wiley & Sons, Inc., Hoboken, New Jersey.

[12] Jian, Linni, Liang, Jianing, Shi1, Yujun and Xu, G.

(2013) "A Novel Double-Winding Permanent Magnet

Flux Modulated Machine for Stand-Alone Wind Power

Generation", Progress In Electromagnetics Research,

142: 275-289.

[13] Chandrasekar, L., Anbuchandran, S. and Sankar, R.

(2015) “Performance Analysis of Pole Amplitude

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Appendix 1: The Generator Data

The studied cage generator is a 3-phase, 10/6.67/5 kW, 380 V, 50 Hz, 4/6/8 pole amplitude

modulated cage induction generator. The parameters of the generator are as follows:

Parameters 4 poles 6 poles 8 poles

R1, Ω 0.47 1.034 1.504

R2, Ω 0.47 0.972 1.361

X1, Ω 0.86 0.7568 0.86

X2, Ω 0.86 0.7568 0.86

Xm, Ω 28.27 23.129 29.55

Rc, Ω 500 409.07 522.710

RT, Ω 0.444 0.968 1.412

XT, Ω 0.8396 0.77 0.899

Stator Rated Current, A 18.44 14.98 13.68

Stator Connection Star Star Delta


ومدار بطاقة الرياح من خلال تعظيم القدرة الخارجة من مولد حث مرتبط بالشبكة التحكم في الأقطاب

عبد الكريم عبد المحسن الغنيم و، محمد عبد السميع عبد الحليم المملكة العربية السعودية، القصيم، أمانة منطقة الفصيم، جامعة القصيمكلية الهندسة،

[email protected]

في هذا البحث تم ربط مولد حثي قفصي بالشبكة الكهربائية، وتم إدارته بطاقة .المستخلصالرياح لتوليد الطاقة كهربائية. استخدم المولد القفصي بدلا من المولد ذي الحلقات الانزلاقية

وهو التحكم في عدد ،غالي الثمن. وتم تحسين أداء المولد القفصي بأسلوب منخفض التكاليفبحيث نعظم الطاقة الميكانيكية المستخرجة من الريح. وفي ،توافق مع سرعة الرياحأقطابه لكي ي

والذي ينتج ثلاث ،هذا الصدد تم التحكم في أقطاب المولد باستخدام تقنية تعديل مدى الأقطابمجموعات من عدد الأقطاب. أثبتت النتائج إمكانية تطبيق هذه الطريق وفاعلية تأثيرها في زيادة

من الدوران عند -وبالتالي توربينة الرياح - حيث مكنت هذه الطريقة المولد ،ج المولدقدرة خر .والتي عندها تستخرج أعلى طاقة رياح ،ا من السرعات المثاليةسرعات تساوي أو قريبة جد

طاقة الرياح، مولد حثي مربوط بالشبكة، تعديل مدى الأقطاب، مولد حثي :كلمات مفتاحية .قفصي

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